I think my sister is too latched onto her son

My sister has always been a bit... Out of her mind. Especially when it comes to her son. When she got pregnant she wanted it just to be her and him but his father wanted to be apart of it and took her to court. For a while they had split custody, but his father died when he was 6. He was found dead in his car.

Every since my sister seems to do whatever to keep my nephew at her side. He's 26 and still lives at home. She would tell him as a good no girl will ever love him as much as his mom and that nobody will ever care about him like she does and he's all she has. He got a gf once and my sister talked about how she hated the girl. Well this girl completely broke his heart. She broke up with him and sent him a message that she found someone else and my sister took the chance to put him down but not actually putting him down. Like saying b"See I told you no girl will love you like mama". My nephew has recently told me about this girl and he thinks he's in love. I've met her and she's so sweet. He said she listens to him and is ok with him talking to much when he went through something she was there. She's a great girl and again my sister is pissed. This has been going on almost 8 months and he is wanting to move in with her and my sister is telling him all girls are nasty whores. When I was over there he said "What if she's different". And she said "She's not. All girls are whores and nobody could love someone like you. Only your mama. Without me you're nothing. Think about Lacey. She broke you because you're nothing and this girl will do the same. I'm the only one who will be here for you". My mom for years told me to stay out of it... But idk if I can anymore...